Cartridge fuse having composite fuse link including ribbon sections and wire sections



` Sept. 22, 1970 E. sALzER 3,530,505 CARTRIDGE FUSE HAVING COMPOSITE FUSE LINK INCLUDING RIBBON SECTIONS AND WIRE SECTIONS l Filed may 25, 1969 l l ....gF...

INVENTOR:

ERWIN SALZER AUnited States Patent O CARTRIDGE FUSE HAVING COMPOSITE FUSE LINK INCLUDING RIBBON SECTIONS AND WIRE SECTIONS l Erwin Salzer, Waban, Mass., assignor to The Chase- Shawmut Company, Newburyport, Mass. Filed May 23, 1969, Ser. No. 827,325 Int. Cl. H01h 85/08 U.S. Cl. 337--159 6 Claims ABSTRACT OF THE DISCLOSURE A cartridge fuse has a fuse link capable on controlling effectively and precisely the maximum arc voltage occurring incident to interruption as well as imparting to the arc voltage a high degree of stability. The fusible element or fuse link of the fuse includes alternating wire sections and ribbon sections both supported on an insulating support. The ribbon sections decrease progressively in Width at the ends thereof adjacent the wire sections and have their smallest width at points of overlap of the ribbon sections by the wire sections, and the ribbon sections have a thickness less than the diameter of the wire sections.

BACKGROUND OF INVENTION There is an urgent need for current-limiting fuses whose current rating is small-from a few amps to fractions of an amp-which have smaller melting i2t values and arcing i2t values than state-of-the-art current-limiting fuses and which are capable of operating effectively under severe interrupting conditions without generating dangerous surge voltages incident to blowing thereof. Such fuses are inter alia required in circuits including critical solid state current control devices, e.g. diodes.

Normally current-limiting fuses for such critical applications include a ribbon type fusible element having a plurality of serially related Ipoints of reduced crosssectional area, referred-to as necks, where the current density is particularly high. Current-limiting fuses for small current intensities may call for ribbon fuse links having a thickness in the order of a few mills, and having necks whose width is in the order of a few mills, and whose cross-sectional area is in the order of a few square mills. Manufacturing such fusible elements is an extremely difficult task which has lbeen unachievable heretofore in many instances. One object of this invention is to achieve this task by simple, inexpensive means.

Another important object of this invention is to provide current-limiting fuses capable of generating arc voltages incident to blowing thereof which are not too high, i.e., so high as to cause any breakdown of any component of the system into which the fuses are included, and which fuses are capable of stabilizing the are voltage so as to minimize clearing iZ-t values and clearing times.

SUMMARY OF INVENTION Fuses according to this invention have fusible elements supported by an oblong strip of insulating material. The fusible conductor means include ribbon sections alternating with wire sections. In order to accelerate backburn and to stabilize the arc voltage initially generated by vaporization of the Wire sections, the ribbon sections decrease progressively in width at the ends thereof ad- 'ice the wire sections. In the structure under consideration the ribbon sections admit large tolerances, or require a relatively small degree of precision and can, therefore, readily be manufactured, eg., by etching or plating methods.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a longitudinal section of a fusible-elementand-support-unit for a fuse embodying this invention taken along 1--1 of FIG. 2;

FIG. 2 is a top-plan-view of the structure of FIG. l;

FIG. 3 is a longitudinal section of another fusible-element-and-support-unit for a fuse embodying this invention taken along IlI-III of FIG. 4;

FIG. 3a is a section taken along Illa-Illa of FIG. 3;

FIG. 4 is a top-plan view of the structure of FIG. 3;

FIG. 5 is a longitudinal section of a complete fuse embodying this invention and including the structure of FIGS. l and 2; and

FIG. 6 is a longitudinal section of a complete fuse embodying this invention and including a modification of the structure of FIGS. l and 2.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and more particularly to FIGS. l and 2 thereof, numeral 1 has been applied to indicate an oblong support of an electric insulating material intended to be arranged in a tubular casing of insulating material substantially in a direction longitudinally thereof, as explained below more in detail. Support 1 supports fragile conductive fusible element means for conductively interconnecting a pair of terminal caps mounted on the ends of the tubular casing housing support 1. The aforementioned conductive fusible element means include spaced ribbon link sections 2 alternating with fusible wire sections 3 having a predetermined diameter, e.g. 3 mills. Wire sections 3 bridge the gaps g formed between contiguous ribbon sections 2. In the embodiment of the invention shown in FIGS. 1 and 2 ribbon sections 2 are arranged alternately on different sides of support 1 and wire sections 3 project through perforations 1a in support 1. The unit shown in FIGS. l and 2 is intended to be submersed in a pulverulent arcquenching filler, preferably quartz sand.

FIGS. 1 and 2 show two ribbon sections 2 on the upper surface of support 1, and one ribbon section 2 on the lower surface of support 1, and two wire sections 3 serially connected with the three aforementioned ribbon sections 2. In a given design the number of serially related ribbon sections 2 and wire sections 3 depends primarily upon the circuit voltage of the circuit in which the fuse is intended to be applied. The higher that voltage, the larger the number of serially connected ribbon sections 2 and wire sections 3.

As shown in FIG. 2, the ribbon sections 2 progressively decrease in Width at the ends thereof adjacent wire sections 3, and ribbon sections 2 have their smallest width at the points of overlap 2a of ribbon sections 2 by Wire sections 3. The thickness of ribbon sections 2 is less than the diameter of Wire sections 3.

The structure of FIGS. 1 and 2 effects a current-limiting interruption of a circuit into which it is inserted when the fault current exceeds the melting izt and/ or vaporization i2-t of wire sections 3. The arc voltage generated by melting and vaporization of wire Sections 3 is not sufficient to effect a satisfactory current-limiting interruption of the faulted circuit. The arc voltage resulting from vaporization of Wire sections 3 decays rapidly as arcing continues, though the arcing time is limited to a small fraction of a half cycle of a current wave of 60 c.p.s. This decay of the arc voltage is, however, compensated by the rapid burnback of the converging ends of ribbon sec- 3 tions 2 immediately adjacent wire sections 3. The speed of burnback of ribbon sections 2 is due to their geometry and their thinness. The rapid burnback of ribon sections 2 results in rapid arc elongation and rapid generation of burnback arc voltage. To be more specific, the particular geometry of the ends of ribbon sections 2 adjacent a wire section 3 coupled with the fact that the thickness of the ribbon sections 2 is less than that of wire sections 3 results in generation of an additional arc voltage that cornpensates largely for the decay of the arc voltage at the initially formed points of break coextensive with wire sections 3.

The structures of FIGS. 3, 3a and 4 includes registering ribbon sections 2' and registering wire sections 3 to both sides of insulating support 1. Insulating gaps having a width g separate adjacent ribbon sections 2' and these gaps are bridged by wire sections 3'. The hottest points of wire sections 3 are situated in the center regions of the gaps formed between ribbon sections 2', and insulating support 1 is provided with perforations 1a coextensive with the hott/est regions of wire sections 3. The arclets resulting from fusion and vaporization of wire sections 3' are separated by a thin layer of quartz sand if support 1 is provided with perforations 1a'. The latter may be deleted if support 1 is sufficiently heat resistant, or the temperature of wire sections is not sufficiently high as long as they carry current to seriously impair support 1.

FIG. 5 shows the structure of FIGS. l and 2 enclosed in a casing 4 of insulating material, e.g., a laminate melamine-glass-cloth. Casing 4 is filled with a pulverulent arcquenching filler 5, e.g. quartz sand, and its ends are closed by a pair of terminal caps 6. Each terminal cap 6 is provided with a slot for the passage of the ends of support 1 and the ends of the two axially outer ribbon sections 2. Each terminal cap 6 defines an outer cavity receiving a solder joint 7 conductively connecting one of the axially outer ribbon sections 2 to one of the terminal caps 6.

FIG. 6 shows basically the same structure as FIG. 5, and the same reference numerals have been applied in both figures to designate like parts. In the structure of FIG. 6 the perforations 1a of support 1 are much larger than in the structure of FIG. 5 and in the structure of FIG. 6 the length of the wire sections 3 by far exceeds the length of the wire sections 3 in the structure of FIG. 5. As a result, the initial arc voltage of the structure of FIG. 6 tends to exceed by far the initial arc voltage of the structure of FIG. 5. In both structures the entire length of wire sections 3 vaporizes substantially at the same time since the cross-sectional areal of wire sections 3 is uniform. The arc voltage resulting from vaporization of wire sections 3 is a steep spike that tends to decay very rapidly. The rapid backburn into ribbon sections 2 tends to effectively counteract this rapid decay of the total arc voltage, i.e., the arc voltage across the arc gaps formed by vaporization of wire sections 3 plus the arc voltage generated by rapid backburn of ribbon sections 2.

When a fuse as shown in FIG. 5 or FIG. 6 is placed in an electric circuit, the number and the size of the wire sections 3 of the fuse being appropriate to the voltage of the circuit, and the fuse is subjected to a short-circuit current, or major fault current, wire sections 3 will melt and vaporize. The resulting arc voltage instantly changes the positive gradient of the rising fault current to negative. The arc voltage spike resulting from fusion and vaporization of wire sections 3 is not sufiiciently high to maintain the current gradient constantly negative until the current becomes Zero. To maintain the gradient of the current permanently negative until the current becomes zero an arc voltage is required in addition to that generated by melting and vaporization of wire sections 3 in the cooling filler medium 5. This additional arc voltage is generated by the rapid backburn of ribbon sections 2.

The structure of FIGS. 1 and 2 may be made of metal- Clad glass-cloth-melamine. The metal layers forming the 4 ribbon sections should be supported on their substratum without interposition of an adhesive therebetween since adhesives tend to adversely affect the interrupting process as, for instance, by tracking. If the fuse is intended to operate at high temperatures the substratum should be an inorganic substance such as a ceramic material. The ribbon sections may be produced by electroless plating of the substratum, etching away the thin metal layer pro'- duced by electroless plating at the areas intended to be nonconductive, and thereafter electroplating the areas intended to be conductive to increase their cross section or thickness. The ribbon sections and the wire sections should preferably be bonded to each other without addition of a bonding metal. The preferred bonding technique depends on the nature of the metals to be bonded to each other and upon the diameter of the wire sections. Ultrasonic welding is desirable welding technique for some applications. In other applications gap-welding is most desirable.

Although this invention has been described in considerable detail, it is to be understood that such description of the invention is intended to be illustrative rather than limting, as the invention may be variously embodied, and is to be interpreted as claimed.

I claim as my invention:

1. A current-limiting fuse including (a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) an oblong support of insulating material arranged inside of said casing and oriented in a direction substantially longitudinally thereof;

(d) conductive means supported by said support and conductively interconnecting said pair of terminal elements, said conductive means including spaced ribbon sections alternating with fusible wire sections having a predetermined diameter and bridging the gaps formed between said ribbon sections, said ribbon sections progressively decreasing in width at the ends thereof adjacent said wire sections and having their smallest width at points of overlap of said ribbon sections by said wire sections, said ribbon sections having a thickness less than the diameter of said Wire sections.

2. A fuse as specified in claim 1 wherein said ribbon sections are supported by said support without interposition of an adhesive therebetween, and wherein said ribbon sections and said wire sections are bonded to each other without addition of a bonding metal.

3. A fuse as specified in claim 1 wherein said support is provided on opposite sides thereof with registering ribbon link sections and registering wire sections.

4. A fuse as specified in claim 1 wherein said support has perforations at the points thereof juxtaposed to said wire sections, the length of said perforations being substantially equal to the spacing between said ribbon sections.

S. In a current-limiting cartridge fuse for a circuit having a predetermined circuit voltage the combination of (a) a tubular casing of insulating material;

(b) a pulverulent arc-quenching filler inside said cas- (c) a pair of terminal elements closing the ends of said:

casing; (d) a plurality of serially connected spaced short sections of wire forming a means for conductively interconnecting said pair of terminal elements, theI number of said sections and the length thereof being insufficient to generate by their vaporization the arc voltage required to force the current to zero without any change of the initial negative gradient thereof to a positive gradient;

(e) a strip of electric insulating material arranged in said casing and submersed in said filler, said strip having a plurality of spaced conductive areas defin- 5 6 ing insulating gaps therebetween, each of said in- 6. An electric fuse as specified in clairn 4 wherein each sulating gaps being bridged by one of said plurality of said plurality of spaced conductive areas has a crossof sections of Wire, said plurality of sections of wire sectional area progressively decreasing in the directions and said conductive areas jointly forming a conductoward each of said plurality of sections of wire. tive interconnection of said pair of terminal ele- 5 ments; and References Cited (f) said plurality of conductive areas being formed by UNITED STATES PATENTS dfctlyladkmg t? ald Strip of les' 3,113,195 12/1963 Kozacka 337- 292 g ma ma Wit out m efposion 0 a 3,236,976 2/1966 Rayno 337-292 adheslve substance, and sa1d overlays having a sufii- 10 3,261,951 7/1966 Jacobs 35,7 292 ciently small cross-sectional area to burnback sufiiciently rapidly in a direction longitudinally of said HAROLD BROOME, Primary Examiner strip to thereby generate the additional arc voltage required to force the current to zero without any U S C1, X R, change of the initial negative gradient thereof from 15 33,7 292 negative to positive. 

